Recently, a switched reluctance motor (hereinafter, SR motor) is applied to a motor for driving a vehicle. The driving of the SR motor is controlled by a controller for feeding electricity including a CPU so as to maintain a target driving speed and a target driving torque. The controller for feeding electricity calculates the target driving speed and the target driving torque on the basis of signals from a shift lever, a brake switch, an accelerator switch and an accelerator opening amount sensor. The controller for feeding electricity outputs a driving control signal to a switching driving circuit on the basis of the result of the calculation and ON/OFF of energization of phase coil of each phases of the SR motor is controlled by the switching driving circuit. Thereby, the driving of the SR motor is controlled so as to maintain the target driving speed and the target driving torque.
FIG. 6 shows an electric circuit for explaining an embodiment of the switching driving circuit which controls ON/OFF of energization of phase coil of each phases of the SR motor on the basis of the driving control signal from the controller for feeding electricity. In FIG. 6, for convenience, a driving circuit for one phase coil is shown.
In FIG. 6, the driving circuit includes, a high electric potential side switching transistor (hereinafter, high electric potential side transistor) 51, a low electric potential side switching transistor (hereinafter, low electric potential side transistor) 52. A collector terminal of the high electric potential side transistor 51 is connected to a plus electric terminal of a battery B via a high electric potential line L1. An emitter terminal of the low electric potential transistor 52 is connected to a minus terminal of the battery B via a low electric potential line L2. A phase coil 53 is connected between an emitter terminal of the high electric potential side transistor 51 and a collector terminal of the low electric potential side transistor 52.
The driving circuit further includes a high electric potential side flywheel diode (hereinafter, high electric potential side diode) 54 and a low electric potential side flywheel diode (hereinafter, low electric potential side diode) 55. The high electric potential side diode 54 is connected between the high electric potential line L1 and the collector terminal of the low electric potential side transistor 52. The low electric potential side diode 55 is connected between the low electric potential line L2 and the emitter terminal of the high electric potential side transistor 51.
The ON/OFF of the high electric potential side transistor 51 and the low electric potential side transistor 52 are controlled with a predetermined chopping frequency in a predetermined control period T, respectively. Thereby, a phase current I feeding to the phase coil 53 of each phases is controlled every control period T and the driving speed and the driving torque of the SR motor are controlled so as to be the target values.
By the way, when the ON/OFF of the high electric potential side transistor 51 and the low electric potential side transistor 52 are controlled at the control period of each phase, an attracting force F is generated between the rotor pole portions and the stator pole portions in the radial direction. Thereby, the surface of the stator is deformed in the radial direction by the attracting force F. As shown in FIG. 5, in the control period T, the attracting force F is gradually increased after the start of the electric feeding control. As a result, the deformation D of the stator is increased in response to the increase of the attracting force F. Then, when the control period T is ended, namely, when the the high electric potential side transistor 51 and the low electric potential side transistor 52 become OFF condition, the attracting force F disappears suddenly. Thereby, the stator resonates and vibrates freely.
The deformation D of the stator generated in the control period T and the vibration of the stator generated just after the end of the control period T are generated in each phases as shown in FIG. 4. The deformation D of the surface of the stator changes largely periodically. The deformation amount ƒ¢D of the deformation D becomes large when the phase is changed and thereby the objectionable acoustic noise is generated.
Driving devices which reduce the objectionable acoustic noise are disclosed in Japanese Patent Application Laid-Open Publications No. 2001-166292 and No. 2001-116183. The driving device disclosed in the Japanese Patent Application Laid-Open Publications No. 2001-166292 decreases the vibration of the stator generated due to the sudden disappear of the attracting force F when the high electric potential side transistor 51 and the low electric potential side transistor 52 become OFF condition. On the other hand, in the driving device disclosed in the Japanese Patent Application Laid-Open Publications No. 2001-116183, an additional vibration which is shifted with a half of cycle with respect to the vibration of the stator generated due to the sudden disappear of the attracting force F is given to the stator and the vibration of the stator is denied.
In the driving device disclosed in the Japanese Patent Application Laid-Open Publications No. 2001-166292, the phase current is slowly decreased when the electric feeding of the phase coils is changed. However, the phase current convergent control period in which the phase current is slowly decreased is long before the phase current reaches the target current. Therefore, although the phase current convergent control period becomes longer when the SR motor rotates under low speed and is able to control, the phase current convergent control period becomes very shorter when the SR motor rotates under high speed and is not able to control. As a result, when the SR motor rotates under high speed, the deformation of the surface of the stator changes largely and periodically and the objectionable acoustic noise is generated.
On the other hand, in the driving device disclosed in the Japanese Patent Application Laid-Open Publications No 2001-116183, as same as the device disclosed in the Japanese Patent Application Laid-Open Publications No. 2001-166292, it is not able to give the additional vibration to the stator and therefore the objectionable acoustic noise is generated. Further, the vibration in the first half cycle of the vibration generated due to the sudden disappear of the attracting force can not be denied when the SR motor rotates under low speed and the objectionable acoustic noise is generated.